To be utterly ridiculous beyond reason. Sometimes used as a verb when shooting tequila like a maniac.
Also refers to someone to whom the party cannot happen without; a Sej is the ultimate party girl.
SEJ is sometimes used to refer to a "Saucy Energetic Jew."

"Dude, I was sejing all night last night, and am totally hurting today."
"Whoa, did you see that Sej over there?!"
"Dude...Sej....enough said!"